The hot metal obtained in the blast furnace or cupola contains considerable quantities of impurities such as sulfur, phosphorus and silicon. Commonly, such impurities are removed after being turned into slag in the ladle or mixer car. Methods and apparatuses that utilize vacuum suction for the removal of slag from the ladles etc. are disclosed in U.S. Pat. Nos. 3,979,108, 4,077,615, 4,160,662 and 4,166,609.
In removing slag by vacuum suction, it is necessary to keep a suction head at a level where it sucks only the floating slag, leaving the hot metal underneath unaffected. This level control is so important that it is usually accomplished by use of oil- and other fluid-based drive units.
Owing to its own weight and that of the connected parts, the suction head is always attracted downward under the influence of gravity. Thus, there has so far been the risk of the suction head dipping in the slag or hot metal when the pressure applied by the level-control drive unit falls or the drive-cylinder piping or hose breaks. Usually, the suction head sprays a large quantity of water from its tip. Entrance of this water into the slag or hot metal is very likely to cause a steam explosion that is highly detrimental to the maintenance of safety.